Plasma display panels are presently in commercial use as digitally addressable information display devices. The panel itself typically consists of two glass plates with a gas mixture sealed between them. A plurality of X-axis electrodes extend in a mutually parallel array on an interior substrate of one plate, and a plurality of Y-axis electrodes extend in a mutually parallel array on the interior of the other plate. The X-axis electrodes are at a 90.degree. angle to the Y-axis electrodes, thereby forming a plurality of intersections between the X-axis and Y-axis electrodes. A typical commercially available AC plasma panel has 512 X-axis electrodes and 512 Y-axis electrodes, yielding 262,144 intersections, or cells.
When a voltage of between 180 to 200 volts is applied across an X-axis electrode and a Y-axis electrode, a discharge in the gas occurs at the cell formed by the electrodes, causing a pluse of light to be emitted at this point. Simultaneously, a charge is collected on the cell wall, which results in the cell being an "on" cell. Once such a discharge has been produced and the cell is turned "on", the collected wall charge acts to continue the discharging when a lesser AC sustain voltage is applied between the electrodes. In an "on" cell, the gas will discharge and the cell will emit a pulse of light at each transition of the applied AC sustain waveform. The sustain voltage, however, is insufficient to initiate a discharge at an X-Y intersection. This phenomonen is known as inherent memory, and was originally disclosed by Baker et al, U.S. Pat. No. 3,499,167, and by Bitzer et al, in U.S. Pat. No. 3,959,190. By precisely timing, shaping, and phasing multiple alternating voltage waveforms supplied to X and Y axes electrodes, the generation, sustaining and erasure of light emitting gas discharges at selected locations on the plasma display panel can be controlled.
Four functions are used to control the operation of an AC plasma panel: the write function, the erase function, the sustain function, and the bulk-erase function. The write function causes a selected cell on the panel to change from the "off", or non-light emitting state, to the "on" or light emitting state. The sustain function maintains the state of all cells on the panel, i.e. causes "on" cells to remain on, and "off" cells to remain off. The sustain function also causes the "on" cells to emit light. The erase function causes a selected cell to be changed from the "on" state to the "off" state. The bulk-erase function causes all "on" cells in the panel simultaneously to be changed to the "off" state.
Operation of the write, erase, sustain, and bulk-erase functions is generally controlled by four logic signals: the X-sustain signal XS, the Y-sustain signal YS, the X-address pulse XAP, and the Y-address pulse YAP. These signals, generally supplied by a waveform ROM (Read Only Memory), are digital pulse trains typically operating at a frequency of 50 kHz. The logic signals are supplied to the sustain and drive circuits, and cause the circuits to execute the four control functions on the panel. Since the typical operational frequency of the plasma display system is 50 kHz, the four control functions operate on a 20 microsecond period.
Currently, the best existing brightness control circuit for an AC plasma panel is that described in "Constant Data Rate Brightness Control For An AC Plasma Panel", by Joseph T. Suste, a copending Patent Application, Ser. No. 273,095, filed June 12, 1981, assigned to the assignee of the present invention, and that specification is hereby incorporated herein by reference. The method used in the Suste application uses a waveform ROM which stores two groups of control signals. The first group performs sustain, write, erase, and bulk-erase functions in a normal manner, emitting two pulses of light per 20 microseconds cycle. The second group of control signals performs the same functions, but with the emission of substantially no light. By mixing the functions from the two groups and varying the ratio of the two groups, a broad range of variable brightness in the operation of the plasma panel is achieved.
Although the Suste brightness control is a great improvement over the prior art, it has several problems. The greatest of these problems occurs when the system is alternately generating brightness control write and brightness control erase functions. During this operation, light will be generated at the rate of 1 pulse per cycle. Therefore, when write and erase functions are being alternately performed, the minimum brightness achievable by the system is 50% of maximum brightness.
This constraint presents a problem, particularly when it is desired to operate the system at the minimum brightness level, 12.5% of full brightness. Sustain pulses at this minimum intensity will provide the correct panel intensity, but if alternate write and erase operations are performed, there will be light flashes four times brighter than this minimum intensity.
A second problem inherent in the Suste brightness control system is that the maximum light the system is capable of producing occurs when the system emits two pulses of light for every 20 microsecond period. The use of plasma panels in areas having a high level of ambient light makes it desirable to increase the number of light emissions per unit of time, to obtain a somewhat higher level of maximum brightness.
It is also desirable to perform 512 cell parallel addressing, which would allow an entire line on a typical 512.times.512 plasma panel be addressed simultaneously. Such parallel addressing is desirable because it increases the data rate of the panel, that is, the maximum rate of writing and erasing the panel, by a factor of 5.3. Since the display characteristics of the plasma panel generally improve as the update rate is increased, particularly in cases where the panel is being used as a video display, a major objective in plasma display panel system design is to increase the update rate as much as possible.
Another problem in the design of brightness control systems for plasma panels is the high intensity discharge of the plasma panel borders. In order to perform write operations on a plasma panel, it is necessary that there be a sufficient number of free particles present in the gas mixture to initiate an avalanche process which causes a write operate to occur. The technique generally used to supply a sufficient quantity of free particles for a write operation utilizes border electrodes around the perimeter of the addressable display area of the plasma panel, which are driven by separate electronic circuits at a sufficiently high voltage level to cause very intense discharges to occur in the border areas. These discharges flood the viewing area of the plasma panel with a sufficient number of free particles to enable the panel to be written with a high degree of accuracy. In past systems, the intensity of light emitted by the border electrodes has not been controllable. In other words, while the viewing area of the panel may be driven so that the brightness level of the display is fairly low, the borders continue to operate at full intensity, and are highly visible to plasma panel observers. It is therefore desirable to reduce the intensity of light generated by these border electrodes.